1. Field of the Invention
The present invention relates to an exhaust manifold for directing an exhaust gas from each cylinder of a multiple cylinder engine to an exhaust pipe.
2. Description of the Related Art
As is known in the prior art, an exhaust manifold is used as an exhaust part for directing an exhaust gas from an engine for an automobile to an exhaust pipe. The exhaust manifold thermally expands because a hot exhaust gas passes from the engine to the exhaust manifold. However, since a flange portion in the exhaust manifold is secured a cylinder head with bolts, the exhaust manifold cannot freely thermally expand, therefore compressive stresses occur at some portions in the exhaust manifold. In particular, the compressive stresses occur at branch pipes secured to the cylinder head and cylinder head side portions in junctions, each junction being formed with two adjoining branch pipes. Japanese Unexamined Utility Model Publication (Kokai) No. 6-76620 discloses an exhaust manifold in which the thicknesses of the branch pipes and the cylinder head side portions are larger than the thicknesses of portions excluding the branch pipes and the cylinder head side portions in the exhaust manifold, so that the branch pipes and the cylinder head side portions can sufficiently withstand the compressive stresses.
As described in Japanese Unexamined Utility Model Publication (Kokai) No. 6-76620, the exhaust manifold of the prior art has a plurality of branch pipes and cylinder head side portions arranged in the longitudinal direction of the cylinder head, in which all of the thicknesses of the branch pipes and the cylinder head side portions are the same. However, the stresses which occur at the branch pipes and the cylinder head side portions are not the same. The stress which occurs at the cylinder head side portion arranged in the middle of the longitudinal direction of the cylinder head is the largest. The stress which occurs at the branch pipe decreases in proportion to an increase of the distance between the branch pipe and the middle of the longitudinal direction of the cylinder head. The stress which occurs at the cylinder head side portion decreases in proportion to an increase of the distance between the cylinder head side portion and the middle of the longitudinal direction of the cylinder head. The stress which occurs at the branch pipe decreases in proportion to a decrease of the distance from the cylinder head to the branch pipe. That is, the thicknesses of the branch pipes and the cylinder head side portions arranged far from the middle of the longitudinal direction of the cylinder head are unnecessarily large, and the thicknesses of the branch pipes arranged close to the cylinder head are unnecessarily large, so that the total weight of the exhaust manifold of the prior art is unnecessarily large.